However. My girlfreind has a Lumia900 withoug 3G on her SIM, so she can only use wifi at home and even when her Phone is just sitting on the table, hasn't been used If she receives a toast (while we are playing a game Wordfeud) she almost Always gets this toats delivered direct. Even when she has'nt touched here Phone for more than 5 minutes!
Could it be that even on the lock screen some app's can wake-up Wifi, or is Wifi behaviour different when you don't have 3G ?

Just wondering, since often people complain that wifi is disabled on the lock screen and WP8 is just getting a WIFI Always on option?

AFAIK, Toast/Push notifications are sent via SMS. For WP7, once the phone locks WiFi is disconnected and the phone receives and sends data via the cellular network. In WP8, after the portico update, users can now set WiFi to remain connected once the phone locks.